Certain signal processing functions used in analog integrated circuits are based on the value of a time constant RC. Typically, the coefficients of an analogue filter or else of an analog digital converter, such as a continuous-time delta-sigma analog digital converter, are achieved by means of resistance R and capacitances C.
The product RC of an analog filter corresponds to its cutoff frequency. For a continuous-time delta-sigma analog digital converter, this time constant RC is related to its sampling frequency. In both these cases, the precision of the time constant RC is important. For the cutoff frequency, if RC is estimated with too high a value, the filter cuts off a part of the signal, and if RC is too small, then the filter does not attenuate the signal sufficiently. In the case of the delta-sigma modulator, it is the performance and the stability of the modulator that depend directly on the precision of the value of RC. The RC coefficients achieve the transfer function of the current-feedback continuous-time delta-sigma analog digital converter.
Depending on the application, the precision with which the product RC must be known is higher or lower. A precision of the value of the time constant RC to +/−5% is generally sufficient.
However, an integrated resistance is accurate to +/−15% while the value of an integrated capacitance varies at least by +/−20% depending on the technology. In conclusion, the time constant RC has a precision of +/−35% which is not sufficient for the great majority of applications.
Various schemes for calibrating a time constant are already known in the prior art. Reference may be made in this regard to the documents U.S. Pat. Nos. 6,169,446, 6,262,603, 6,803,813 and 7,078,961.
The calibration described in these documents is based on a comparison of the value of the time constant defined by the product RC with a known and accurate time base formulated, for example, by a quartz-driven clock, or on a comparison of the voltage across the terminals of the resistance R with that present across the terminals of the capacitance C when they are traversed by the same current.
These schemes for calibrating the time constant RC merely deliver a digital word which directly controls a resistance or a variable capacitance.
Moreover, they are all sensitive to the analogue imperfections intrinsic to the hardware components used to make the converter, such as the offset voltage of an operational amplifier or of the comparators used.
Moreover it is known, in the prior art, to use a current-feedback continuous-time delta-sigma analog digital converter coupled to a filter at input receiving a sinusoidal input signal, so as to calibrate the time constant of another current-feedback continuous-time delta-sigma analog digital converter.
However, the use of a second current-feedback continuous-time delta-sigma analogue digital converter is expensive in terms of time and cost.
In view of the foregoing, it is proposed to circumvent most of these limitations related to the calibration of a time constant by carrying out a measurement of a time constant of an integrated circuit of increased precision.